By Sue Kendall
PREVIEW columnist
By early 2020, a new virus called SARS-CoV-2, the cause of the COVID-19 disease, was spreading rapidly across the planet. Everyone wanted and needed information about what it was, what it did, and how it was transmitted.
Since then, the global scientific community has gathered a lot of information about the virus and the Agricultural Research Service (ARS) contributes to that knowledge base.
Scientists are continuing to research whether the virus was originally transmitted from animals to humans. Researchers at the ARS Arthropod-Borne Animal Diseases Research Unit (ABADRU) in Manhattan, Kansas, are working with Kansas State University to investigate whether insects are in any way involved in the transmission of this virus. The team recently published two papers outlining their studies and results.
In March, the first study was published online in the Journal of Medical Entomology. It focused on biting insects, particularly mosquitoes and two types of mosquitoes, as they are known to ingest disease-causing viruses and transmit them to humans and animals. The insects were allowed to feed on SARS-CoV-2-riddled blood, which is a natural route of infection, and were later tested for the presence of viral RNA and infectious virus at various times. Several lines of insect cells were also tested to see if the virus could multiply in them.
The team found that virus replication was not supported in any of the insects or cell lines tested.
“We conclude that these biting insects pose no risk of transmitting SARS-CoV-2 to humans or other animals after a blood meal infected with SARS-CoV-2,” said ARS research molecular biologist and study author Dana Mitzel.
The second study, published in April in the journal Parasites and Vectors, focused on houseflies because of their association with the mechanical transmission of bacterial, parasitic, and viral pathogens to humans and animals. The team wanted to see if houseflies could acquire SARS-CoV-2 and transmit it to their surroundings.
The houseflies were exposed to SARS-CoV-2-spiked culture media or virus-spiked milk. (The team used reconstituted milk powder as a substrate because it also serves as a food source for raising the flies, and they knew the insects would easily feed on it.) The flies were tested for viruses four and 24 hours after exposure. All flies exposed to the virus tested positive for viral RNA, but infectious virus was only present in the flies exposed to the milk fortified with the virus.
In the next experiment to check the transmission, the flies were again exposed to milk containing the virus and tested positive for the virus 24 hours later. However, the flies could only mechanically transfer viral RNA, not infectious virus, to unvaccinated (clean) milk and test surfaces. However, while some flies appeared to be able to transmit infectious virus, they could not transmit the virus, only the virus’s RNA, which is not infectious.
“We want to emphasize that this research was conducted under carefully controlled laboratory conditions with high levels of virus – much more than what flies might encounter naturally. More research is needed to see if house flies can transmit infectious SARS-CoV-2 in a natural environment and, if so, what are the public health implications, ”said ARS entomologist Dana Nayduch, research director at ABADRU and correspondent Author of the study. “As we were able to detect both viruses and viral RNA in flies, it is on a positive note that we may be able to use flies to monitor and detect viruses in the environment. In this type of surveillance strategy, the housefly does all the work by visiting animals, including humans, and their litter. Then we can examine flies in the laboratory for signs of the virus and know if it’s out there in places the flies have visited. “
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